FR2831005A1 - Short optical pulse generator for optical communication has sample grating distributed Bragg reflector - Google Patents

Abstract

A short optical pulse generator (1) has a Sample Grating Distributed Bragg Reflector (40) with gratings (31, 33, 35, 39) and lines (32, 34, 36, 38) with integral reflection frequency steps equal to multiples of the reflection modes in a waveguide (3) cavity (50) coupled to the end (13) of the semiconductor mode locking laser active layer (7).

Description

C: \ rEMP \ text_dôtôt.doc

  BRIEF OPTICAL PULSE GENERATOR

DESCRIPTION

  Technical Field The invention relates to the field of optical pulse generators comprising a laser active part inside a resonant cavity delimited by a first reflector and a second reflector which is a Bragg reflector, the cavity having a plurality of resonant modes, each mode

  resonant corresponding to a mode optical frequency.

  PRIOR ART There are several solutions for creating short optical pulses using a semiconductor laser. These solutions can be classified in

three categories.

  A first category of solutions uses

  Gain switching semiconductor lasers.

  This solution is widely used because of its simplicity. The widths of the pulses produced are of the order of 15 picoseconds. The pulses are

  strongly modulated in wavelength ("Chirpées").

  A second category of solutions uses

  absorption or Q-switched lasers.

  A saturable absorbent is included in the structure of the laser. External modulation or passive switching of the absorption makes it possible to obtain

brief impulse.

  A third category of solutions uses

  mode-locked lasers.

  This technique is very widespread. It currently gives the best results in terms of pulse width and in terms of repetition frequency. Thus it was obtained with mode locking lasers with 40 GHz repetition frequency, see article [1] whose reference appears in

  annex to this description. I1 was also obtained

  pulse widths as short as 180 fembeseconds, see article [2] with reference

  is appended to this description.

  Although under the above conditions the results cited above could be obtained, mode-locked lasers are usually used under conditions such that only a fraction of the gain band of the semiconductor medium is used, typically less than 5 nanometers while the usable band is of the order of 40 to 100 nanometers. In this way, relatively large impulses are obtained, generally greater than 0.5 picoseconds. In addition, the jitter, that is to say the temporal drifts from one pulse to another, (jitter) is important in particular for the pulses.

less wide.

Brief description of the invention.

  Compared to the prior art which has just been described, the invention relates to a pulse generator of optical pulses, with mode locking semiconductors capable of generating pulses of very short duration with a stable repetition frequency. The invention aims to obtain this result in a simple and reproducible manner so as to limit the

production costs.

  According to the invention, provision is made to use a mode-locked semiconductor laser coupled to a sampled Bragg grating reflective waveguide. The mode-locked semiconductor laser is known, coupled to a reflective waveguide with a Bragg reflective network. In such a laser a resonant Fabry Perot cavity is formed between a rear reflecting face of the cavity and the distributed Bragg reflector. According to the invention instead of using a simple Bragg reflective network,

  a sampled Bragg reflective network is used.

  In itself known such a network consists of a plurality of identical sections, each consisting of a part of uniform waveguide without Bragg network, and a part of uniform waveguide with Bragg network . It has been shown that such a network generates multiple reflection peaks, each reflection peak corresponding to a reflection optical frequency value, see article [3], the reference of which is annexed to

  present description. If more the point of reflection

  of the sampled guide is positioned so as to constitute with the rear face of the laser a cavity the longitudinal dimensions of which induce resonant modes of said cavity for said reflection frequencies, it is emitted simultaneously over a wide frequency spectrum and consequently it is possible to generate optical pulses of very small width. The sampling period of the sampled guide, i.e. the length between two consecutive sragg gratings, allows the value of the repetition frequency of the laser pulses to be adjusted.

  emitted by the laser thus formed.

  In summary, the invention relates to an optical pulse generator including a laser active medium inside a resonant cavity delimited between two reflectors, a first and a second, the cavity having values of mode optical frequencies for which the cavity is resonant, the second reflector being constituted by a Bragg reflector, characterized in that said Bragg reflector is constituted by a Bragg grating sampled each sample of the grating having several reflection peaks, each peak corresponding to a frequency value reflection optics, the difference between consecutive values of reflection frequencies being equal to an integer multiple P of the difference between consecutive values of frequency

fashion.

  The value of the integer multiple notably includes the unit value so that the difference between consecutive values of reflection frequencies can also be equal to the difference between values

  of optical mode frequencies.

  The integer multiple P can naturally take any value greater than or equal to 1, such as 1, 2, 3 and more. A choice of values greater than 1, such as for example 2 or 3 allows spacing to be used

  smaller resonant modes of the Fabry cavity-

  perot and thus greater lengths of the active medium. This way we can get bigger

output powers.

  In a real i sat ion mode, the Bragg reflective network is modulated in wavelength. and

or engraving depth.

  The waveguide can be an optical fiber coupled to the active medium. It can also be integrated on the same chip as an active part of the chip including the active medium, a medium for propagation of the guide being located in the extension of the active medium

  forming the active layer of said active part.

  In one embodiment, the semiconductor medium is made up of two parts, a first and a second located in the extension of one another, each of the two parts being included in a section, each of the sections having a control input. its own. In this embodiment, the first section can, for example, be a gain section. The second section can for example be a phase tuning section. The effective group index of the semiconductor medium of said second phase tuning section being adjustable by electro-optical effect by changing a value of an electrical quantity applied to said second section. The second section can also be for example a section whose semiconductor medium is

  consisting of an absorption layer.

Brief description of the drawings.

  Embodiments of the invention will now be described with reference to the accompanying drawings in which: FIG. 1 is a diagram representing a general embodiment of the invention, FIG. 2 represents a sampled network

  written on a waveguide or a fiber.

  FIG. 3 is composed of three parts A, B. C, representing on the same frequency scale, in part A the spacing of the reflection peaks of the sampled Bragg reflector, in part B the frequency values corresponding to resonant modes of the cavity formed between the rear face and the Bragg reflector, and in part C the emission spectrum of the laser pulses emitted by the laser

according to the invention for P = 1.

  FIG. 4 schematically represents an embodiment of a part of a laser according to the invention in which a guide layer is included in a first part which is a gain section and in a second part which is a tuning section phase, the two guide parts being placed in the

extension of one another.

  FIG. 5 schematically represents an embodiment of a part of a laser according to the invention in which a guide layer is included in a first part which is a gain section and in a second part which is an absorption section , the two guide parts being placed

  in line with one another.

  FIG. 6 schematically represents an embodiment of the invention in which the sampled network is produced on an integrated waveguide

  on the same chip as the active part.

  FIG. 7 schematically represents a part of a wavelength-division multiplex network in itself known using an integer P of

  laser generators according to the invention.

  FIG. 8 represents on a frequency axis the frequency combs of a parts of an integer P of lasers according to the invention used in a telecommunications network multiplexed in wavelength.

  Description of embodiments

  Figure 1 is a diagram showing a mode

  general embodiment of a laser 1 according to the invention.

  It represents an active part 20 having the structure of a semiconductor laser, for example with surface ruhan. In a manner known per se the active part 20 comprises an active layer 4. The active layer 4 is arranged in a manner known per se in electrical and optical confinement layers not referenced in the figures. A metallic contact layer 10, for example made of AuPt, is located above the confinement layers. A Fabry-Perot cavity 50 is formed in two parts coupled to each other. A first part 7 of the cavity 50 of

  Fabry-Perot is constituted by said active layer 4.

  A rear face 13 ′ of this layer is cleaved or treated to have a high reflection coefficient. A front face 13 of this active layer is treated to have a reflection coefficient as low as possible and if possible zero. The material of the active layer 4 is chosen to have an amplification gain over a large bandwidth and to be capable of receiving direct modulation at high frequency, for example 10 or gigahertz or more. A waveguide 3, for example as shown in FIG. 1, an optical fiber, is arranged in the longitudinal extension of the active layer 4. The waveguide 3 is optically coupled to the laser layer 4, preferably in this example embodiment, by a coupling lens 14. A sampled distributed network Bragg reflector 40 (SGDBR, Sample Grating Distributed Bragg Reflector) is etched on the waveguide 3. The sampled reflector 40 (Sample Grating, SG) by alternating samples of distributed Bragg re ective resectors, (Distributed Bragg

  Reflector, DBR) 31, 33 ... and sections 32, 34 ....

  each consisting of a waveguide part without samples from the Bragg grating. The rear reflective face 13 ′ and the Bragg reflector 40 form

  together the Fabry-Perot cavity 50.

  The operating mode of the laser generator 1 according to the invention as shown in FIG. 1 and details on the Bragg reflector 40 will now be explained and commented on, in conjunction with the

Figures 2 and 3.

  The sampling period ZO represented in FIG. 2 is defined as the distance separating the start of two consecutive zones 31, 33 gravoes. Each etched region has a length Z1 and is etched with a lattice pitch A. In a known manner, such a network has multiple reflection peaks. The frequency spacing AvSG between two consecutive peaks of reflection is given by the formula: AvSG = c / 2ngfZO (1) In this formula: c is the speed of light in a vacuum, ngf is the group index of the fiber or the guide. The sampling period of the guide ZO is chosen to obtain a frequency spacing AvSG between two consecutive reflection peaks, corresponding to the repetition frequency of the pulses which it is desired to have generated by the laser according to the invention. For example for a repetition frequency of 40 gigahertz it will take a period of about 2.5 mm in

  a silica fiber having a group index of 1.5.

  The central wavelength B of the periodic Bragg grating engraved on each zone 3i, 33 35 of length Z1 is a function of the grating period and of the index n of the guide or of the fiber according to the formula: B = 2nA (2) The spectral width of the envelope of the reflection comb formed by all of the reflection peaks is determined by the length Z1 of each of the engraved zones. The Bragg B wavelength, the frequency spacing AvSG between two consecutive reflection peaks and the number of reflection peaks can thus be precisely controlled by choosing for the realization of the network the value of parameters such as the length ZO of the periods of the network, the length Z1 of each of the engraved parts, the pitch of each of the engraved parts, and the length L of the network

sampled.

  The optical cavity 50 formed between the rear reflecting face 13 ′ (FIG. 1) of the active layer 4 and the sampled network reflector 40 of length L, has a length equal to the sum of the length La of the active part 4 and of the length L1 between the entry of the guide and the effective point of reflection of the light. In such a cavity, the AvFP spacing between two consecutive longitudinal resonant modes is given by the formula: AvFP = c / 2 (naLa + ngfL1) (3) in which: na and La respectively represent the value of the group index and the length of the active layer 4 between its rear 13 ′ and front 13 ′ faces, L1 is the length between the end of the guide or of the fiber 3 facing the active part 4, and starting in the example commented on in connection with FIG. 1 at the lens 14 and the point of this fiber at which the light reflected by the rear reflecting face 13 ′ is effectively reflected by the

sampled reflector 40.

  It will have been recognized in the formula (3) above that (naLa + ngfL1) represents the optical length of the optical cavity formed between the rear face 13 'and the Bragg reflector 40 when the small distance between the lens 14 and the front face 13 of the active part 4. Naturally if this optical distance is not negligible, it will have to be taken into account. The length L1 can be determined by measuring the propagation time of an optical signal in the

sampled network.

  FIG. 3 which will now be commented on is composed of three parts A, B. C, representing on the same frequency scale, partly at the spacing of the reflection peaks of the sampled Bragg reflector, part B the frequency values corresponding to resonant modes of the cavity formed between the rear face and the Bragg reflector, and in part C the emission spectrum of the pulses

  laser emitted by the laser according to the invention.

  In the example commented on in connection with FIG. 3, the sampled reflector Bragg network 40 is etched on the fiber 3 so that the spacing AvSG of the reflection peaks given by the formula (1) above is equal to the spacing AvFP given by formula (3) above, resonant modes of the Fabry Perot cavity formed between the rear face 13 ′ of the active part and the point of the waveguide 3 where the light is effectively reflected. This fact is represented in FIG. 3 parts A and B where it can be seen that the reflection peaks of the guide 3 represented in A and the resonant modes of the cavity represented in B are located one under the other, thus showing that these peaks and these resonant modes correspond to the same frequency values. The case represented in FIG. 3 corresponds to the case where P is equal to 1. In this case the components of the emission spectrum of the laser 1 are also located at the same frequencies as shown in part C. As explained above, the fiber 3 can also be etched so that the AvSG spacing of the reflection peaks given by formula (1) above is equal to another integer multiple of the AvFP spacing given by formula (3) above. In this case for the same cavity the spacing of the modes will be as represented in part B. On the other hand if we take for example P = 2, a peak in two no longer appears in part A, the frequencies corresponding to the resonant modes of the cavity 50 shown in part B do not change since they only depend on the optical length of the cavity, one frequency out of two in part C is

  deleted, because the corresponding mode is not excited.

  Preferably, in order to maintain the operating mode during an entire operating sequence of the laser according to the invention, there are means in themselves known, not shown in the figures, for stabilizing the temperature of the

  part 20 comprising the active layer 4.

  If a frequency modulation f such that f = AvSG = PAvFP (4) is applied to the active part, the modes are locked, which means that the excited modes will be synchronized in phase. The number of resonant modes excited in the Fabry-Perot cavity is equal to the number of reflection peaks of the sampled Bragg reflector 40, this provided that the gain curve of the active medium has a bandwidth at half height greater than that of the envelope of the reflection curve of the sampled network. The greater the number of peaks, the smaller the width of the pulses produced. As an example if the length of the Fabry-Perot cavity and the reflector 40 are designed to have the same spacing value of 40 gigahertz between resonant modes of the cavity and between consecutive peaks, and if there are 10 reflection peaks, the bandwidth Av of the transmitted signal is 400 gigahertz. A Fourier transform of this signal from frequency space to time space leads under these conditions to a width of modulation At of the signal of At = 0.43 / Av = 1 picosecond We thus see that the only limit to the brevity of impulels come from the inter-band gain dynamics in the semiconductor material constituting the

active layer 4.

  With such a configuration the fluctuations in the repetition frequency are reduced because the spacing of the peaks of the reflection comb of the segmented reflector can be very well controlled and the

  spectral width of each peak can be very narrow.

  Network implementation variants

  will now be described.

  The network of each of the samples can have a variable pitch (chirped) or be apodized, that is to say that the depth of the etching can be modulated according to the longitudinal direction of the network. When the network is with variable pitch, the pitch can vary in a linear or non linear fashion with the longitudinal distance with respect to a fixed point of the guide. The advantage of such a variable pitch network is as explained in article [4], the reference of which is annexed to the

  present description, that one increases the stability

  Modal. When the network is apodized the coupling coefficient between the outward and the return wave of each period can vary so as to eliminate lateral reflection peaks in the curve giving the value

  of the reflection as a function of the optical frequency.

  Finally, according to another alternative embodiment, a subharmonic modulation can be applied. In this case the active region or as will be described later, a phase modulation section of this region is modulated with a frequency whose value is a quotient by an integer of the difference between the frequencies of modes corresponding to resonant modes excited from the Fabry-Perot 50 cavity. This reduces the stress on the need for high frequency electronics as well as on the band

modulation modulation pass.

  Alternative embodiments of the active part 20 will now be commented on in connection with FIGS. 4-6. In these figures, the elements having the same function as those commented on in connection with FIG. 1 have the same reference number. These elements having the same function will not

  nocally commented again.

  FIG. 4 represents an embodiment in which a semiconductor guide layer 4 ′ including the active medium is formed in two coupled parts 7, 8 situated in the extension of one another. The first part 7 of the guide layer 4 ′ is a gain part, for example with multiple quantum wells formed by a first epitaxy. The second part 8 of the guide layer 4 ′ is a wave guide part having an electro-optical effect, for example of Franz-Keldysh, formed by a second selective epitaxy. Each of the parts 7, 8 of the guide layer is integrated on a single substrate 9 made of InP. The part of the laser is formed in this case in two sections 5, 6 on the single substrate 9. The two sections differ

  from each other by the nature of the guide layer 4 '.

  The first part 7 of the guide layer 4 'constitutes the active layer of the first section 5. The second part 8 of the guide layer 4' is included in the second section 6. The two parts 7, 8 of the guide layer are arranged in itself known manner in electrical and optical confinement layers not referenced in the figures. A metallic contact layer 10, for example made of AuPt, is located above the confinement layers. The two sections 5, 6 are separated from each other by an etching 12 carried out in the contact layer 10 and in a part of the confinement layers located above the guide layer 4 '. As a result, the two sections 5, 6 can each receive their own command, for example in the form of a current injected into the first section 5 and in the form of a voltage applied to the second section 6. It is thus possible to change the optical length of the waveguide 4 ′, by a voltage command applied to the second section 6, without appreciable influence on the power emitted by the

laser.

  Thus we separate the gain function devolved to section 5 and the phase tuning function devolved to section 6. We can thus obtain relatively easily an excellent agreement between the spacing of the reflection peaks which results from the realization of the Bragg network on the guide and the spacing of the resonant modes of the Fabry-Perot cavity 50 which depends on the optical length of the cavity 50 which can be varied in an independent manner by acting on a value of command in section 6. The command value in this section 6 can also be used as a modulation command to produce a modulation frequency. In that case

  the laser is locked by frequency modulation.

  Thanks to frequency modulation, each frequency component is in phase with the other frequency components. (Frequency modulation mode-locked) According to another real i sat ion mode which will now be discussed in connection with FIG. 5, the active part 20 is formed as in the previous case described in conjunction with FIG. 4 formed in two sections 5, 6 '. The first part 7 of guide layer 4 "of the first section 5 is, as in the previous case, a gain layer for example with quantum wells. The second part 8 'of guide layer 4" of

  the second section 6 'is an absorption layer.

  This second part 8 ′ of the guide layer may for example be of quantum wells or of bulk material. The polarization of the absorption section 6 ′ is, as in the previous case, independent of the polarization of the gain section 5. The polarization of the absorption section 6 ′ can be either a negative voltage applied to the n / p junction between the guide layer 8 ′ and an electrical confinement layer, or a low current. The second part 8 'of guide layer 4 "can be enriched with ions for

  shorten the lifespan of carriers.

  The configuration described in connection with FIG. 5 allows passive mode locking by

  passive switching of the absorption section 6 '.

  This operating mode allows in particular an all clock optical re-activation. In this operating mode the pulses produced by the switching of the losses are synchronized in frequency

  and in phase with those of the injected signal.

  An embodiment in which the 3 ′ waveguide carrying the sampled Bragg reflector 40 is formed in a guide integrated into the chip carrying the active part 20 will now be described in conjunction with

Figure 6.

  For manufacturing facilities, it is preferable to make the sampled network on a fiber or on a silica or polymer waveguide as shown in FIGS. 1, 4 or 5. In the case of the fiber or the waveguide in silica or polymer samples of Bragg rescues 31, 33 can be made by direct exposure. In particular in the case of fibers or guides made of silica, in addition to the well-known advantages of these guides, low losses, well-controlled technologies, reliability, it should be noted that the refractive index of these guides can be adjusted continuously. by a procedure of uniform illumination of the guide. This adjustment of the refractive index value allows fine adjustment

  the spacing of the AvSG reflectivity peaks.

  The sampled network can also, as shown in FIG. 6, be etched on a waveguide integrated in InP. In this case, as shown in this figure, the waveguide part 3 ′ is formed on the chip carrying the active part 20, in a guide layer 11 having one end 15 abutted to the guide layer 4 of the part 20. The part 20 can be used according to any of the modes commented on in connection with FIG. 1 or with FIGS. 4 or 5 as shown in FIG. 6. It is however noted that in this embodiment, we are currently limited to lasers operating at very high repetition frequencies, for example greater than 80 gigahertz, this due to the limited length that can currently be

  obtain with such guides on semiconductors.

  Lasers according to the invention can advantageously be used for the realization of

  wavelength multiplex telecommunications.

  (WDM). In this case, the central network frequency of the sampled reflector assigned to each laser and the envelope of the reflection peaks of the reflector will be chosen so that there is no overlap between the reflection combs of the various lasers. FIG. 7 shows such a multiplexed network, in itself known composed of N - N designating an integer - laser generators 1 according to the invention references 1-1 to 1-N. Each of the generators 1 is coupled to a modulator 2. The modulator units 2 are referenced 2-1 to 2-N. The N modulators are coupled to a multiplexcur 30. FIG. 8 shows on a frequency axis the different reflection spectra of a first laser according to the invention, the central network frequency of which is set on a first frequency f1, and that of a second and an Nth laser respectively having a central network frequency f2, fN. The different center frequencies fl, f2, fN correspond to

ITU grid frequencies.

Annex

  list of relevant prior art documents.

  [1] D. Arbel, U. Koren, S. Shalom, M. Winik, L. Reznik, M. Zimmermann, R. Sasson, D. Hadas, S. BreitUar, 27 th conference on optical communications, September 30-October 4, 2001; Netherland, Text of the

conference not yet published.

  [2] R. Ludwig, S. Diez et al, IEICE Trans. Electron., Vol

E81-C, n 2, Pages 140-145, 1998.

  [3] V. Jayaraman, Z-M. Chuang and L. A. Goldren, IEEE Journal of Quantum Electronics, vol. 29, no 6, pages

1824-1834, 1993.

  [4] A. Morton, V. Mizrahi, et al Applied Physics

  Letters, Vol. 64, Pages 2634-2636, 1994.

Claims (10)

RESELL I CAT I ONS   1. Generator (1) of optical pulses including a laser active medium (4) inside a resonant cavity (50) delimited between two reflectors (13 ', 40) a first (13') and a second ( 40), the cavity (50) having optical frequency values of modes for which the cavity is resonant, the second reflector (40) being constituted by a Bragg reflector, characterized in that said Bragg reflector (40) is constituted by a Bragg grating sampled each sample (31, 33) of the grating (40) having several reflection peaks, each peak corresponding to a value of optical frequency of reflection, the difference between consecutive values of frequencies of reflection being equal to one integer multiple P of the difference between values   frequency of modes.   2. Generator (1) of optical pulses according to claim 1 characterized in that the network (40)   Bragg reflector has a variable pitch.   3. Generator (1) of optical pulses according to claim 1 or 2 characterized in that the network (40) Bragg reflector is modulated in etching depth in a longitudinal direction of the network (40). 4. Generator (1) of optical pulses according to   one of claims 1 to 3 characterized in that the   waveguide is an optical fiber (3) coupled to active medium (4).   5. Generator (1) of optical pulses according to   one of claims 1 to 3 characterized in that the   waveguide (3 ') is integrated on a chip including the active laser medium (4), a medium (8) for propagation of the guide being located in the extension of said medium active (4).   6. Generator (1) of optical pulses according to   one of claims 1 to 5 characterized in that   a guide layer (4 ′) including the laser active medium (7) is made up of two parts, a first (7) and a second (8) located in the extension of one another, each of the two parts (7, 8) belonging to a section (5, 6, 6 '), each of the sections (5, 6, 6')   having its own command entry.   7. Generator (1) of optical pulses according to claim 6 characterized in that the first section (5) is a gain section, and in that the second section (6) is a phase tuning section, l effective group index of said second part (8) of the guide layer included in the phase tuning section (6) being adjustable by electro-optical effect by changing a value of an electrical quantity   applied to said second section (6).   8. Generator (1) of optical pulses according to claim 6 characterized in that the first section (5) is a gain section, and in that the second part (8 ') of the guide layer (4 ") included in the second section (6 ') is a layer absorption.   9. Optical telecommunication system wavelength multiplexed, characterized in that it includes pulse generators (1-1 - 1-N) optics according to claim 7.   10. All-optical device for recovering a clock signal, characterized in that it comprises a generator (1) of optical pulses according to the